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Story: Eels

Where do freshwater eels come from? Aristotle claimed they sprang straight from river mud, and some people thought they grew from bits of skin, or even horsehair. In 1896 an Italian discovered that a tiny ‘saltwater fish’ was in fact the larva of the freshwater eel. We are gradually learning more about their remarkable life journey, which begins and ends in the ocean.

Story Summary

Eels look like snakes, but they are fish, with tiny scales on their slimy skin. There are 15 freshwater species worldwide. New Zealand has three species. They are top predators: no other species prey on them.

Longfin eel

These eels are named for their back fin, which is longer than on the shortfin eel. Dark brown to black, they live in rivers and streams throughout the country. These are probably the world’s biggest freshwater eels, and are New Zealand’s largest native fish. They can grow up to 1.75 metres long, and the largest one ever caught weighed 24 kilograms.

One study found that males swam out to sea to breed at an average age of 23 years, and females at an average age of 34. They die after reproducing. At Lake Rotoiti near Nelson, some are over 100 years old.

Shortfin eel

This smaller native eel is olive green and lives closer to the coast. It can grow to 1 metre and weigh up to 3.5 kilograms. Males reproduce and die at about 14 years, females at about 22.

Spotted eel

This is a recent arrival, and may have come from Australia. It was first noted in New Zealand in 1997. Its back is blotchy, and it lives only in northern rivers.

Feeding

Eels mainly search for food at night, using their sense of smell, and sensors in their skin. They eat worms, insects, grubs, snails and fish. Researchers found that one huge eel had eaten a whole duck.

The eel mystery

For centuries, people did not understand the eel’s life cycle, because larval (baby) eels look nothing like adult eels. We now know that eels have different forms during their life. But it is still unclear where New Zealand eels breed – it is probably thousands of kilometres away in the tropical sea.

Life cycle of New Zealand eels

Eels spawn (lay eggs) in warm seas. The female lays millions of eggs, and the male fertilises them. The adults then die.

Leaf-shaped larvae hatch and drift to New Zealand’s coast.

The larvae change shape and become see-through eels (glass eels). Between July (winter) and December (summer), but mostly in spring, they migrate in huge numbers into river mouths.

Glass eels then turn grey-brown and become elvers. They swim up rivers, mostly at night. Sometimes they slither across grass, and they can climb up waterfalls.

Elvers get bigger and become adult eels, living in rivers, lakes and streams. They live there for many years before they breed. In autumn (February–April) they swim downriver and out to sea to breed.